﻿<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        html, body {
            height: 100%;
        }

        .box {
            height: 100%;
            border: 0.5px solid red;
            background-color: #ccc;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>